1. Field of the Invention
This invention relates to polyurethane elastomers and is more particularly concerned with hydrolytically stable polyester polyurethane elastomers.
2. Description of the Prior Art
The preparation of thermoplastic polyurethane elastomers from polyester polyols is well known in the art. Such elastomers can be extruded, injection molded, cast and fabricated in other known manner. Generally speaking, such polyester based polyurethane elastomers are recognized for their greater resistance to high temperatures as compared to their polyether polyol based counterparts. However, the latter polyether based polyurethanes enjoy a recognized advantage in hydrolytic stability over the polyester materials.
Hydrolytic instability of polyester polyurethanes, or, their loss in physical properties when exposed to hydrolyzing conditions such as the combination of high temperature and humidity, has long been recognized in the art. Also receiving long recognition has been the concept of relying on carbodiimide containing additives both reactive and non-reactive types to impart enhanced hydrolytic stability to various polymer systems including, particularly, polyester polyurethanes. Presumably, the carbodiimide additives function in the polymer by virtue of the quick reaction of the carbodiimide linkage with a water molecule to form a urea thereby preventing polymer molecular weight reduction which would otherwise occur via scission of polymer links by aqueous hydrolysis of the ester functions.
Typical of the prior art methods for stabilizing polyester polyurethane elastomers is the use of Isonate.RTM. 143L as the diisocyanate in the preparation of one-shot elastomers which are not thermoplastic. The 143L liquid diisocyanate is a proprietary product which contains; carbodiimide linkages which, in turn, impart the hydrolytic stability to the final polymer; see Upjohn Tech. Bull. 402, June 1, 1969 entitled "The Use of Isonate.RTM. 143L in Urethane Elastomers". It should be noted that the polyester polyurethanes obtained are not thermoplastic.
Further, it is known to employ carbodiimide containing isocyanates in the preparation of cast polyester polyurethanes; see U.S. Pat. No. 4,088,665, example 17.
U.S. Pat. No. 3,193,522 teaches the stabilization of vulcanized or cross-linked polyesters and polyester urethanes through the addition of polycarbodiimide additives having more than 3 carbodiimide groups in the polycarbodiimide chain, and, optionally, the presence of isocyanate groups also in the polycarbodiimide chain which would allow the chemical incorporation of the polycarbodiimide into the polyester urethane.
U.S. Pat. No. 3,226,368 discloses the preparation of polyester polyurethanes wherein excess isocyanate over and above that required to react with the polyester polyol is concomitantly reacted in the presence of a carbodiimide forming catalyst to form a large number of carbodiimide groups in the polyurethane chain. However, this method of production is not suitable when a fast type of polymerization process is required, typically, when the polyurethane is being prepared in a screw extruder or the like. The gas evolving carbodiimide formation would create bubbles or voids in the polymer as it was being extruded. In fact, this particular reference at column 1 beginning at line 24 seeks to avoid accelerated reactions and specifically calls for a slow reacting system which would be suitable for casting techniques where prolonged pot life is required.
We have now discovered how to prepare thermoplastic polyester polyurethanes which are characterized by a high degree of hydrolytic stability when tested under rather severe conditions of humid aging.
Further, because the carbodiimide entity which imparts hydrolytic stability is polymerized into the polyurethane chain it cannot be solvent leached or extracted from the polymer. Contrastingly, the majority of the prior art methods providing for enhancement of hydrolytic stability of polyester polyurethanes are not leach resistant (see the comparison of sample 4 versus samples 7 and 8 set forth in Example 2 below).
Surprisingly, the physical properties of the polyester polyurethanes of the present invention, particularly tensile strengths and elongations, are superior to prior art polyester polyurethanes having large amounts of polycarbodiimide prepared in situ during polyurethane formation or those having additive carbodiimides (see U.S. Pat. Nos. 3,226,368 and 3,193,522 cited supra) to achieve solvent resistant hydrolytic stability.
Most surprising, however, is the fact that the present thermoplastic polyester polyurethanes have all of the above features combined with the fact that the polymers are completely clear with transparent clarity. The combination of all of these properties, i.e. thermoplasticity, hydrolytic stability, solvent resistance, superior tensile strength and elongation, and very good polymer clarity, has not hitherto been found in one thermoplastic polyester polyurethane to our knowledge.